This project will deal with a physico-chemical investigation of the properties of the enzyme porcine heart mitochondrial malate dehydrogenase. Recent work by this laboratory has implicated the essentiality of 4 residues in the active center of this enzyme, i.e., (histidine, cystine, lysine and arginine). A tryptic peptide containing the active center histidine has been purified and sequenced yielding information pertaining to those amino acids (12) in close linear proximity to the active center histidine. A tryptic peptide containing the active center cystine has been purified and its amino acid composition reported. Sequence studies will be performed on peptides derived from labeled peptides containing these residues in order to identify other residues in near linear proximity. Bi- functional reagents will be utilized in order to implicate the proximity of each of these active center residues to one another in solution as opposed to a static situation as observed in x-ray work. The objective of this work will be attempt to propose and to understand the function of each of these residues in the overall mechanism of action of this enzyme.